1. Field of the Invention
The present invention relates to a control device and a control method for a hybrid vehicle. More particularly, the present invention relates to a control for a hybrid vehicle including an engine and a motor-generator connected to the engine through a power split device.
2. Description of the Background Art
As one example of hybrid vehicles, there has been proposed a hybrid vehicle including an engine, a power split device, a first motor-generator and a second motor-generator. Herein, the power split device is a planetary gear mechanism in which a carrier is connected to a crank shaft of the engine and a ring gear is connected to a transmitting member coupled to a drive shaft. The first motor-generator is connected to a sun gear of the planetary gear mechanism. The second motor-generator can transmit/receive mechanical power to/from the drive shaft.
For example, Japanese Patent Laying-Open No. 2005-045863 discloses a control method for a hybrid vehicle configured as described above. According to this control method, even when a rotational speed of an electric motor (second motor-generator) or a generator (first motor-generator) is in a lockup area, the motor-generator can pull out of the lockup state promptly, so that a drive circuit of the motor-generator is prevented from being overheated. According to this control method, more specifically, if the motor-generator is in the lockup state, the rotational speed thereof pulls out of the lockup area promptly with the use of a set lockup state pull-out torque. Herein, the lockup state pull-out torque is set larger as a lockup state retention time becomes longer.
Moreover, Japanese Patent Laying-Open No. 2006-187169 discloses another hybrid vehicle configured as described above. In this hybrid vehicle, a first motor-generator and a drive circuit thereof are protected in consideration of restriction on a torque to be required for a drive shaft and restriction on transmission/reception of power to/from an electric storage device. Specifically, a rotational speed of an engine is restricted for restriction on a torque so as to suppress an unexpected torque for a driver from being transmitted to a drive shaft and for restriction on transmission/reception of power to/from an electric storage device. Then, the rotational speed of the engine is further restricted for suppressing a first motor-generator from being driven at a rotational speed of almost zero. Next, a target rotational speed of the engine is set in consideration of theses restrictions on the rotational speed. Thus, the engine and two motor-generators are controlled.
According to the control for pull-out of the lockup state by the control method disclosed in Japanese Patent Laying-Open No. 2005-045863, the lockup state pull-out torque to be set herein is larger than a normal value in order to cause the motor-generator to pull out of the lockup state. However, if the produced torque can not be increased any more with ease, for example, if the torque produced by the first motor-generator already reaches a maximum torque, the motor-generator can not pull out of the lockup state.
For example, in a state where the first motor-generator is operated in a low-speed and regenerating mode and the second motor-generator is operated in a high-speed and power running mode, the hybrid vehicle is further accelerated. As a result, the rotational speed of the first motor-generator shifts from a normal rotation area to a reverse rotation area. Herein, the rotational speed of the first motor-generator passes over 0 rpm in a state where the first motor-generator produces a maximum torque. In other words, the rotational speed of the first motor-generator enters the lockup state (extremely low-speed area). In such a case, however, the control for pull-out of the lockup state disclosed in Japanese Patent Laying-Open No. 2005-045863 fails to cause the first motor-generator to pull out of the lockup state.
Moreover, Japanese Patent Laying-Open No. 2006-187169 discloses a technique for restricting an operating point of the engine, in particular, a rotational speed of the engine so as to avoid occurrence of the lockup state, but does not disclose a technique for causing the motor-generator in the lockup state to pull out of the lockup state promptly.